The present disclosure relates to a thin film transistor using oxide semiconductor for a channel layer, and a display device and an electronic unit, which use the thin film transistor.
Recently, research and development of oxide semiconductor such as zinc oxide or indium-gallium-zinc oxide have been actively conducted with the aim of applying the oxide semiconductor to electronic devices such as a thin film transistor (TFT), a light emitting device and a transparent conductive film. As generally known, when such oxide semiconductor is used for an active layer (channel) of TFT, the TFT has high electron mobility and thus has an excellent electric characteristic compared with TFT using amorphous silicon, which is typically used for a liquid crystal display or the like. In addition, the TFT may be advantageously expected to have high mobility even at low temperature near room temperature, and therefore the TFT is being actively developed. As such TFT using the oxide semiconductor layer, TFT having a bottom-gate or top-gate structure has been reported (for example, see WO2005-088726).
A known bottom-gate TFT is structured such that a gate electrode is provided on a substrate, and a thin film layer of oxide semiconductor is formed on the gate electrode via a gate insulating film (for example, see Japanese Unexamined Patent Application Publication No. 2007-194594). Such a structure is similar to a structure of currently commercially used, bottom-gate TFT using amorphous silicon for a channel. Therefore, an existing manufacturing process of the TFT using amorphous silicon may be easily used for manufacture of TFT using oxide semiconductor, and therefore commercialization of the TFT using oxide semiconductor for a channel is gradually progressing.
However, since the oxide semiconductor is not high in heat resistance, oxygen or zinc may be eliminated during heat treatment in a manufacturing process of TFT, resulting in formation of lattice defects, as generally known. The lattice defects cause electrically shallow impurity levels, leading to reduction in resistance of the oxide semiconductor layer. Therefore, use of oxide semiconductor for a channel of TFT leads to normally-on operation, where certain drain current flows though gate voltage is not applied, or depression operation. Consequently, threshold voltage is reduced with increase in defect levels, leading to increase in leakage current. Furthermore, as generally known, similar impurity levels are caused by mixing of a particular element such as hydrogen in addition to the above impurity levels caused by lattice defects (for example, see Cetin Kilic et. al. “N-type Doping of Oxides by Hydrogen” APPLIED PHYSICS LETTERS, 81, 1, 2002, pp. 73-75).
Therefore, a transfer characteristic of TFT has been disadvantageously changed during a manufacturing process or the like, leading to shift of threshold voltage of the TFT in a negative (minus) direction.
For example, when oxide semiconductor is used to form an n-type channel, electron concentration in the channel increases, as a result, threshold voltage of TFT tends to have a negative value. For the TFT using oxide semiconductor, since a p-type channel is hard to be formed, only n-type TFT needs to be used for circuit formation. In such a case, when the threshold voltage has a negative value, a circuit configuration becomes undesirably complicated.